Thursday, October 28, 2010

Did Galápagos Turtle Lineage Survive Ancient Blast?

John Roachfor National Geographic News

October 2, 2003

About 100,000 years ago the top of Volcano Alcedo in the Galápagos Islands exploded in a violent eruption that smothered the region in pumice and blew away all but one lucky lineage of the giant tortoises that lived there, according to a new study.

"The only lineage that probably survived the eruption was the one that repopulated the region," said Luciano Beheregaray, a molecular ecologist at Macquarie University in Sydney, Australia.

The Galápagos tortoises are perhaps the best known animals from the island archipelago off the Pacific coast of South America. They can grow as heavy as 550 pounds (250 kilograms) and live over 100 years.

The tortoises are believed to belong to one species (Geochelone nigra) with 14 different subspecies spread throughout islands, including three species that have been hunted to extinction since humans arrived in the 1600s.

Beheregaray is part of a research group that exploits advances in genetics to study the evolution and distribution of tortoises in the Galápagos. Their most recent paper appears in the October 3 issue of Science.

Other members of the group include Claudio Ciofi, Adalgisa Caccone, and Jeffrey Powell at Yale University in New Haven, Connecticut; Dennis Geist at the University of Idaho in Moscow; and James Gibbs at State University of New York in Syracuse.

According to the new research, the tortoises that live on Volcano Alcedo on Isabela Island today are descendants of the lone surviving lineage of the volcanic catastrophe, and they carry a record of the eruption in their genes.

"Our study shows that we can infer specific historical events in a population using DNA markers from extant individuals," said Beheregaray.

Christopher Schneider, a biologist at Boston University in Massachusetts, said that the use of genetic markers to infer population history has been around for more than a decade, but that estimates for when events took place are often quite broad.

"However, we don't want to throw the baby out with the bathwater because sometimes broad estimates are useful, so I hope that theoreticians keep hammering away on this problem," he said.

Genetic Analysis

Today the 3,000 to 5,000 tortoises that make up the Alcedo tortoise population (Geochelone nigra vandenburghi) are the largest group in the Galápagos, but they have very low genetic diversity compared to four other populations on Isabela. Each population there occupies a different volcano.